The present invention relates to a head for the linear dimension checking of mechanical pieces, including a casing, that defines a longitudinal geometric axis, an arm-set, movable with respect to the casing, a feeler, coupled to the movable arm-set, for touching the piece to be checked, a bias device, arranged between the casing and the movable arm-set, a detecting device, coupled to the casing, including a movable element, and a transmission device, between the movable arm-set and the movable element of the detecting device, including a wire, substantially rigid in said longitudinal direction but flexible in the directions perpendicular to the longitudinal direction, the wire having a first end coupled to the movable arm-set and a second end for cooperating with the movable element of the detecting device.
Contact detecting and measuring heads are known from U.S. Pat. No. 5,299,360.
The mechanic structure of these heads, based on a coupling between the movable arm-set and the casing, achieved by means of a first constraining system substantially of the cone-ball type and a second constraining system for preventing rotations of the movable arm-set about the longitudinal geometric axis, and on the presence of two annular facing surfaces adapted to enter into a substantially point-to-point contact, guarantees good repeatability.
However, in some applications, it is required that the repeatability errors be as small as possible, considerably smaller than 1 xcexcm.
The repeatability errors are mainly due to the effects of frictions among reciprocally moving parts, to shape errors of elements of the head, and to vibrations due to shock waves or other dynamic phenomena.
This is particularly true with regard to the detecting device and its arrangement in the head. More specifically, a detecting device disclosed in U.S. Pat. No. 5,299,360 includes a microswitch with a stem having an end adapted for contacting the movable arm-set. This contact may involve slidings and thus repeatability errors. Moreover, as the stem of the microswitch is not constrained with respect to rotations about its axis, and it is possible that there may be errors in the shape of the stem, other repeatability errors may consequently possibly arise.
From the article xe2x80x9cMulti-directional probexe2x80x9d published on pages 7 and 8 of the publication xe2x80x9cTechnical Digest No. 6, April 1967xe2x80x9d of the American company Western Electric, there is known a measuring probe with a housing, that defines a frusto-conical cavity, a position transducer measuring device fixed at the interior of the housing, and a movable arm-set with shaped portions for providing contact and a frusto-conical portion for cooperating with the frusto-conical cavity of the housing. The measuring device includes a movable element coupled to an end of a wire, axially rigid but flexible in transversal directions. The other end of the wire is connected to the movable arm-set. Moreover, the measuring device includes means, not shown, that presumably consist of an internal spring, that, by means of the movable element and the wire, urge the frusto-conical portion of the movable arm-set towards the frusto-conical cavity of the housing.
Therefore, the wire is subject to a compressive stress of a considerable entity, owing to the fact that it has to apply force for causing contact between the movable portion of the arm-set and the cavity of the housing. Consequently, problems arise insofar as the dimensions of the section of the wire are concerned, because of the contrasting requirements of providing resistance to a considerable compressive stress on the one hand and achieving good lateral flexibility on the other. Moreover, even the frictional forces acting on the wire necessarily have relatively high values.
A tracer head including a wire (or rod spring) and the 10 other features according to the technical field of the present invention is shown in U.S. Pat. No. 4,187,614. This head includes a stylus adapted to touch a model surface and a tracer shaft carrying the stylus. The shaft includes a rigid portion and a rod spring portion, respectively coupled to a casing by means of a first and a second mutually parallel planar springs. Axial and radial movements of the stylus are transmitted to an axial and, thanks to the bending of the rod spring, two radial displacement detectors coupled to the casing. The connection of the shaft by means of the two planar springs causes a compression action on the rod spring, in particular when the stylus touches the model surface, with consequent problems, e.g. in connection with the choice of its sectional dimensions, as cited above with reference to the article xe2x80x9cMulti-directional probexe2x80x9d.
Object of the present invention is to provide a checking head in which the component of the repeatability error due to the detecting device is particularly small.
This and other objects are achieved by a checking head of the herein described type in which the bias device is arranged in such a way that it urges the movable arm-set into contact with the casing and does not apply a compressive stress onto the wire of the transmission device.
The invention enables to achieve negligible frictional forces in the transmission device and in the detecting device, avoid problems insofar as the design sizes of the transmission device are concerned and, in conclusion, considerably improve the head repeatability.